A system for producing liquefied natural gas and method

ABSTRACT

The system comprises a natural gas feed and a natural gas liquefaction facility having a refrigeration circuit comprising: a compressor adapted to compress at least one refrigerant fluid; a driver adapted to drive the compressor; and a heat exchanger adapted to receive a flow of natural gas from the natural gas feed and remove heat therefrom by heat exchange against the refrigerant fluid. A heat pump collects low-temperature thermal energy rejected from the natural gas liquefaction facility and transfers the collected thermal energy to a thermal energy storage system at a higher temperature. The system further comprises a processing facility powered by thermal energy from the thermal energy storage system.

TECHNICAL FIELD

The present disclosure concerns natural gas liquefaction systems and methods.

BACKGROUND ART

Natural gas, mainly composed of methane and including minor amounts of heavier hydrocarbons, such as ethane, propane, butanes, pentanes, hexanes, benzene, toluene and others, as well as possibly carbon dioxide, is becoming an increasingly popular source of energy, due to its large availability and reduced environmental impact. However, transportation of natural gas from the site of extraction, where the source of natural gas is available, to the end user, is a main concern. One cost-effective and safe method of transporting natural gas over long distances is to liquefy the natural gas and to transport it in tanker ships, often referred to as LNG carriers. At destination the liquefied natural gas is transformed back in its gaseous state and made available to the end users.

Different systems and technologies have been developed to liquefy natural gas (NG) and produce liquefied natural gas (LNG), using single or combined refrigeration circuits, wherein one or more refrigerant fluids are cyclically compressed and condensed in a condenser, expanded and heated in heat exchange relationship with natural gas and/or with another refrigerant fluid. Thermal energy is removed from the natural gas by the refrigerant fluid and rejected in the environment through the condenser, where compressed refrigerant is chilled and condensed by heat exchange against a cold source, e.g. air or water.

Several different natural gas liquefaction systems and methods have been developed over the decades and are known to those skilled in the art.

Natural gas liquefaction is an energy consuming process and continuing efforts are being made to improve the overall efficiency of LNG production systems.

Specifically, energy is needed to drive the compressors required to compress the refrigerant(s). Additional energy is required by ancillary facilities and units of the natural gas liquefaction system such as, but not limited to, gas pre-treatment facilities. These are required to remove undesirable components from the raw natural gas coming from the gas field, prior to liquefaction. Components to be removed from the raw natural gas include, for instance: heavy hydrocarbons (HHC), such as pentane and heavier hydrocarbons, benzene, toluene, xylene; mercaptans; hydrogen sulfide (H₂S); carbon dioxide; and moisture (H₂O).

Several natural gas pre-treatment facilities are used to remove one or more of the above-mentioned undesired components. By way of example, such pretreatment facilities include, but are not limited to, sweetening facilities, adapted to remove carbon dioxide and other acid gas components (hydrogen sulfide, mercaptans, and the like), de-hydration facilities, adapted to remove moisture, optionally fractionation systems or the like, adapted to remove and recover heavy hydrocarbons.

There is a continuous need for further improving the energy efficiency of natural gas liquefaction systems and methods.

SUMMARY

According to an aspect, disclosed herein is a system for producing liquefied natural gas, comprising a natural gas feed and a natural gas liquefaction facility having a refrigeration circuit. The refrigeration circuit comprises a refrigeration compressor adapted to compress at least one refrigerant fluid, a driver adapted to drive the refrigeration compressor, a cooler to remove heat (thermal energy) from the refrigerant fluid during or after compression, and a heat exchanger configured to receive a flow of natural gas from the natural gas feed and remove heat therefrom by heat exchange against the expanded refrigerant fluid.

The system for producing liquefied natural gas further includes a thermal energy storage system adapted to receive and store therein thermal energy rejected by the natural gas liquefaction system at a low temperature and recovered by means of a heat pump. The heat pump is adapted to collect low-temperature thermal energy rejected from the natural gas liquefaction system and transfer the rejected thermal energy to the thermal energy storage system at a higher temperature, i.e. at a temperature higher than the temperature at which the thermal energy is rejected.

Furthermore, the system may comprise at least one processing facility, powered by thermal energy from the thermal energy storage system. In some embodiments, the thermal energy stored in the thermal energy storage system may be used for power generation, i.e. thermal energy can be delivered to a thermodynamic cycle which converts thermal energy into mechanical energy, subsequently converted into electric energy.

As used herein, the term “refrigerant” or “refrigerant fluid” is any fluid capable of undergoing thermodynamic transformations of compression, cooling and expansion, in order to extract heat from the natural gas to be liquefied and to reject thermal energy removed from the natural gas.

In many LNG systems, one or more refrigerant fluids, different from the natural gas itself, are used in various combinations and are processed in closed loops, i.e. closed cycles. Some LNG systems, however, use a flow of liquefied natural gas as the refrigerant fluid in an open loop, i.e. in an open circuit, without requiring additional refrigerant fluids, different from the natural gas itself. The novel features disclosed herein can be employed in both kinds of LNG systems, as will be illustrated below, with reference to some exemplary embodiments.

The thermal energy, which is recovered through the heat pump, may be part of the thermal energy rejected from the refrigerant fluid during or after compression and prior to expansion thereof.

In addition, or alternatively, in some embodiments, the recovered low-temperature rejected thermal energy may include heat rejected from one or more processing facilities of the LNG system, different from the refrigeration circuit. For instance, rejected heat can be recovered by the batch-wise operating regeneration of a dehydration unit provided for removal of moisture from the raw natural gas prior to liquefaction in a natural gas pre-treatment facility.

The energy efficiency of the LNG system is improved, as at least part of the rejected thermal energy is not released in the environment, but rather collected in the thermal energy storage system for use by an ancillary processing facility requiring thermal energy for the operation thereof, for instance, or for other uses, such as electric power generation.

The heat pump allows storage of the thermal energy at a temperature higher than the temperature of rejection, making the thermal energy more valuable for use in thermodynamic cycles. The power required to drive the heat pump is less than the useful power recovered through the heat pump.

For instance, the natural gas liquefaction system may include one or more pre-treatment facilities, adapted to receive and pre-treat natural gas prior to deliver the natural gas to the natural gas liquefaction facility. Pre-treatment requires thermal energy, which can be entirely or at least partially provided by the thermal energy storage system.

In some embodiments, the driver which drives into rotation the refrigeration compressor or compressor train of the refrigeration circuit may include an electric motor. The electric motor can be powered by electric power generated by an electric generator driven by a thermal energy conversion system adapted to convert thermal energy into mechanical energy and to drive the at least one electric generator therewith.

The thermal energy conversion system can comprise an internal combustion engine, such as in particular a gas turbine engine, fueled with natural gas directly or indirectly delivered by the natural gas feed. In advantageous embodiments, a waste heat recovery unit adapted to recover waste heat from the internal combustion engine and further adapted to transfer waste heat to the thermal energy storage system can be provide, for further improving the energy efficiency of the system.

The LNG system may further include a carbon dioxide capturing facility adapted to receive flue gas from the internal combustion engine and remove carbon dioxide therefrom. Thermal energy required to operate the carbon dioxide capturing facility can be at least partly provided by the thermal energy storage system.

Further embodiments and additional features of the system according to the present disclosure are set forth here below with reference to the enclosed drawings.

According to another aspect, the present disclosure relates to a method for liquefying natural gas with a natural gas liquefaction system.

The method includes flowing natural gas in a heat exchanger of a natural gas liquefaction system and removing thermal energy from the natural gas by heat exchange against a refrigerant fluid. Low-temperature thermal energy is removed from the refrigerant fluid through a refrigerant cooler. Moreover, the method comprises the step of recovering low-temperature thermal energy rejected from the natural gas liquefaction system and transferring the rejected thermal energy to a thermal energy storage system through a heat pump at a temperature higher than the temperature at which the thermal energy has been rejected.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is now made briefly to the accompanying drawings, in which:

FIG. 1 is a diagram of one embodiment of a natural gas liquefaction system;

FIG. 2 is a diagram of one embodiment of a natural gas liquefaction system using electric energy to drive the refrigeration compressor of the natural gas liquefaction facility;

FIG. 3 is a more detailed diagram of a further embodiment of a natural gas liquefaction system;

FIG. 4 is a more detailed diagram of a yet further embodiment of a natural gas liquefaction system;

FIG. 5 is a flowchart of a method according to the present disclosure; and

FIGS. 6, 7, 8, 9, 10, 11, 12 and 13 are schematic diagrams of alternative natural gas liquefaction facilities, which may be used in combination with a heat pump and a thermal energy storage system of the present disclosure.

DETAILED DESCRIPTION

In general terms, disclosed herein are systems and methods adapted to improve the overall efficiency of a natural gas liquefaction system, wherein thermal energy (heat) rejected by the natural gas liquefaction system is exploited to reduce the amount of energy required to operate the system. Specifically, part of the low-temperature thermal energy removed from the natural gas during the liquefaction process is recovered and used for various purposes, instead of being released in the environment. The recovered low-temperature thermal energy is transferred to a thermal energy storage system at a temperature higher than the temperature at which it is rejected, using a heat pump. The heat pump can be driven by electric energy also used to drive the refrigeration compressor of the refrigeration cycle. The overall energy efficiency of the system is thus increased, since low-temperature thermal energy is exploited for operating one or more thermal energy requiring facilities of the system. The ecological footprint of the LNG production system can be reduced.

In addition to recovering thermal energy from the refrigerant of one or more refrigeration cycles, low-temperature thermal energy can be recovered from any location of the natural gas liquefaction system, where thermal energy is rejected at a temperature sufficiently above ambient temperature.

When multiple refrigeration circuits are used, rejected thermal energy can be recovered from at least one of the refrigeration circuits. In embodiments, one or more refrigeration circuits may include more than one compressor or compressor stage. Thermal energy can be recovered between two sequentially arranged compressors or compressor stages, through an intercooling heat exchanger, for instance, or downstream the last compressor, or compressor stage, possibly upstream of a refrigerant condenser.

In general, not the entire rejected thermal energy is recovered, but only a fraction thereof, while a residual fraction of thermal energy at near-to-ambient temperature can be rejected in the environment.

While recovering thermal energy from the refrigerant after compression thereof is particularly beneficial, thermal energy can be recovered also from other sources of rejected thermal energy. A suitable source of recoverable low-temperature thermal energy can be the pre-treatment section of the LNG system, at the outlet of a reactor and/or drier regeneration system, for instance.

Certain exemplary embodiments will now be described to provide an overall understanding of the principles of the structure, function and use of the systems, devices and methods disclosed herein. One or more examples of these embodiments are illustrated in the accompanying drawings. Those skilled in the art will understand that the systems, devices and methods specifically described herein and illustrated in the accompanying drawings are non-limiting exemplary embodiments and that the scope of the present invention is defined solely by the claims. Features described or illustrated in connection with one exemplary embodiment may be combined with the features of other embodiments. Such modifications and variations are intended to be included within the scope of the present invention.

Referring now to the drawings, FIG. 1 illustrates a somewhat simplified diagram of a system 1 for production liquefied natural gas according to embodiments disclosed herein. The system 1 comprises a natural gas feed 3, feeding raw natural gas (RNG) to the system 1. The raw natural gas RNG can be pre-treated in a gas pretreatment facility 5. While for the sake of brevity reference will be made herein to a single gas pre-treatment facility 5, those expert in the field will understand that a plurality of systems or facilities may be foreseen, depending upon which kind of undesired components are to be removed from the raw natural gas RNG. For instance, the gas pre-treatment facility 5 can in turn include a sweetening system, a gas dehydration system, a heavy-hydrocarbons (HHC) removal system, a natural gas liquids (NGL) removal system, or combinations thereof.

After removal of undesired components such as acid gas (carbon dioxide, hydrogen sulfide, mercaptans), heavy hydrocarbons and moisture therefrom, pretreated natural gas (NG) is delivered from the gas pre-treatment facility 5 to a natural gas liquefaction facility schematically shown at 7, including a refrigeration circuit. The natural gas liquefaction facility 7 can be configured as any natural gas liquefaction facility known in the art. By way of example, but without limitation, the natural gas liquefaction facility 7 can be based on using any suitable liquefaction process. By way of example only, and not limitation, any of the following types of liquefaction processes could be used: single refrigeration cycles, such as nitrogen cycles and single mixed refrigerant cycles, double refrigeration cycles, such as propane-mixed refrigerant LNG cycles, double mixed refrigerant cycles, three refrigerant cycles, or others.

The natural gas liquefaction facility 7 may include one or more refrigeration circuits in combination, with one or more heat exchangers adapted to bring the natural gas flow into heat exchange relationship with one or more refrigerant fluids, depending upon the kind of natural gas liquefaction facility 7 used. In general terms, the natural gas liquefaction facility 7 includes at least one compressor arrangement, comprised of one or more compressors, adapted to compress, i.e. pressurize, at least one refrigerant fluid. A driver arrangement drives one or more compressors into rotation. The driver arrangement may in turn include one or more drivers. The compressed refrigerant fluid is cooled and condensed in a condenser arrangement, which may in turn comprise one or more condensers adapted to remove heat from the compressed refrigerant fluid. A refrigerant expansion arrangement is further included in the natural gas liquefaction facility 7 and adapted to expand the cooled and condensed refrigerant(s). A heat exchanger of the natural gas liquefaction facility 7 is configured to receive a flow of natural gas and remove heat therefrom by heat exchange with the expanded refrigerant fluid, wherein the heat exchanger can include one or more heat exchanger units.

The structure, arrangement and operation of the various components of the natural gas liquefaction facility 7 can vary and largely depend upon the gas liquefaction technology used. For the sake of the present disclosure, what matters is that the natural gas liquefaction facility 7 includes a compressor arrangement comprised of one or more compressors and relevant driver(s), condensers, expanders and heat exchangers in combination, to cause one or more refrigerant fluids to undergo cyclic thermodynamic transformations, whereby thermal energy is removed from the natural gas NG flowing through the LNG production system 1 and rejected therefrom.

For the sake of simplicity, the natural gas liquefaction facility 7 is represented in FIG. 1 simply as a combination of a refrigerant compression system 9 that can receive power 11 from a suitable power source, to be described, a condenser or other refrigerant cooler 13 adapted to reject thermal energy (heat) from the refrigerant fluid processed in the refrigerant compression system, an expander 15 and a heat exchanger 17 to remove heat from the natural gas NG and liquefy the natural gas. Refrigerant fluid compressed by the compression system 9 is cooled and condensed in condenser 13 by heat extraction therefrom, expanded in expander 15 and caused to flow in heat exchanger 17 in heat exchange relationship with natural gas NG to remove heat from the natural gas.

Thermal energy transferred in heat exchanger 17 from the natural gas NG to the refrigerant fluid, is removed from the refrigerant fluid in condenser 13. As will become apparent from the following description, according to various embodiments the natural gas liquefaction system 1 may include more than one condenser, and/or different refrigerant coolers in combination, wherefrom heat extracted from the natural gas is rejected.

Liquefied natural gas LNG is collected in a liquefied natural gas storage and offloading facility 19 including an LNG storage tank 22 and a cryogenic pump 20.

To improve the energy efficiency of the system 1, a thermal energy storage system 21 is provided, wherein thermal energy rejected by the natural gas liquefaction facility 7 is collected at a temperature higher than the rejection temperature, i.e. the temperature at which the thermal energy, or part thereof, is rejected from the natural gas liquefaction facility 7.

A heat pump 23 driven by an electric motor 25 is provided to recover low-temperature thermal energy rejected from the refrigerant fluid and transfer said thermal energy at a higher temperature in the thermal energy storage system 21. A low-temperature heat transfer fluid circuit 23A is provided between the condenser 13 and the cold side of the heat pump 23, and a high-temperature heat transfer fluid circuit 23B is provided between the hot side of the heat pump 23 and the thermal energy storage system 21. Heat transfer fluids circulate in the respective circuits 23A, 23B by means of pumps (not shown).

In one implementation the heat pump 23 can be a trans-critical heat pump.

Mechanical power generated by the electric motor 25 is used to transfer the thermal energy from a lower temperature at the condenser 13 to the higher temperature at the thermal energy storage system 21.

The electric motor 25 and the compression system 9 can be powered by an electric distribution grid as described in greater detail later on with reference to the following figures.

Instead of rejecting the thermal energy removed from the natural gas in the environment, as in LNG systems of the current art, said thermal energy is at least partly collected and stored in the thermal energy storage system 21 at a temperature suitable for use in other processing facilities of the system 1.

In the exemplary embodiment of FIG. 1 , the thermal energy storage system 21 provides at least part of the thermal energy required for operating the gas pretreatment facility 5. A heat transfer fluid circuit 27 circulates a heat transfer fluid from the thermal energy storage system 21 to the gas pre-treatment facility 5 and from this latter back to the thermal energy storage system 21. If the thermal energy storage system 21 is not able to cover the entire heat demand of the gas pre-treatment facility 5, the option is not excluded of providing an additional heat generator (not shown), or else to use heat from an additional source of heat available in or near the system 1.

The natural gas liquefaction and heat recovery method performed by the system 1 of FIG. 1 is summarized in the flow-chart of FIG. 5 . Specifically, a first step (201) includes introducing a flow of natural gas into the heat exchanger of the liquefied natural gas production facility 7. A second step (202) includes removing low-temperature thermal energy from the flow of natural gas. A third step (203) provides transferring the thermal energy at a higher temperature through the heat pump 23 to the thermal energy storage system 21. Finally, a fourth step (204) includes delivering thermal energy from the thermal energy storage system 21 to the at least one processing facility.

With continuing reference to FIG. 1 , a further embodiment of a system 1 for producing liquefied natural gas is shown in FIG. 2 . The same reference numbers of FIG. 1 designate the same or equivalent parts, components or elements in FIG. 2 , which will not be described again.

In FIG. 2 cooling and condensation of the refrigerant fluid are achieved using two cooling or condensing units 13, 13A sequentially arranged downstream of the delivery side of a compressor arrangement 47 of the compression system 9. In embodiments, the first unit 13 can be a cooler which removes a fraction of thermal energy at a higher temperature from the refrigerant fluid, while the second unit 13A can be an actual condenser, wherein further thermal energy is removed at a lower temperature until the refrigerant fluid is condensed.

Heat rejected at the cooler 13 is recovered via heat pump 23, while heat rejected at the condenser 13A is rejected in the environment. In general, therefore, only a fraction of the rejected thermal energy from the refrigerant fluid is recovered from the compressed refrigerant fluid(s) used in the natural gas liquefaction facility 7. The amount of thermal energy recovered through the heat pump 23 and the amount of thermal energy rejected in the environment depend upon the temperature in the low-temperature side of the heat pump 23 and upon the temperature at which the refrigerant fluid condenses.

In FIG. 2 an electric energy distribution grid 29 is shown, which powers the electric motor 25 of the heat pump 23. In the embodiment of FIG. 2 , the electric energy distribution grid 29 further powers a driver, such as an electric motor 31, which drives the refrigeration compression system 9. An electric connection between the electric energy distribution grid 29 and the cryogenic pump 20 of the LNG storage and offloading facility 19 is further shown in FIG. 2 . The pump 20 loads the liquefied natural gas from the storage tank 22 in an LNG carrier (not shown), for instance.

In one implementation, electric energy can be generated by an electric generator 33 driven by a thermal energy conversion system 35, herein also referred to shortly as a thermodynamic system 35, which converts thermal power into mechanical power. The thermodynamic system 35 may include an open thermodynamic cycle, such as a Bryton cycle, using a gas turbine engine. The gas turbine engine may be fueled with natural gas NG from the natural gas feed 3. In other embodiments, the thermodynamic system 35 may include a closed thermodynamic cycle, such as a Rankine cycle using water or an organic fluid as a working fluid.

In embodiments, the thermodynamic system 35 may include a combined top, high-temperature cycle and a bottom, low-temperature cycle, for instance a high-temperature gas turbine cycle, the waste heat whereof is used to power a bottom Rankine cycle, e.g. a steam or organic Rankine cycle.

As schematically depicted in FIG. 2 , the thermodynamic system 35 is in heat exchange relationship with the thermal energy storage system 21 through a heat transfer circuit 37, in which a heat transfer fluid circulates. Thermal energy can be transferred from the thermodynamic system 35 to the thermal energy storage system 21 or vice-versa. For instance, low-temperature thermal energy can be delivered from the thermal energy storage system 21 to a low-temperature thermodynamic cycle in the thermodynamic system 35 and converted therein into mechanical energy to drive the electric generator 33.

With continuing reference to FIGS. 1 and 2 , a more detailed diagram of a further embodiment is shown in FIG. 3 , wherein the same reference numbers designate the same or corresponding elements shown in FIGS. 1 and 2 .

The system 1 of FIG. 3 comprises a natural gas feed 3, feeding raw natural gas (RNG), which is pre-treated in a gas pre-treatment facility 5. After removal therefrom of undesired components such as acid gas (carbon dioxide, hydrogen sulfide, mercaptans), heavy hydrocarbons and moisture, pre-treated natural gas (NG) is delivered to a natural gas liquefaction facility 7.

As noted above, the natural gas liquefaction facility 7 is represented herein in a simplified manner, as including a simple refrigeration circuit. In embodiments, the natural gas liquefaction facility 7 may include one or more closed or open refrigeration circuits in combination, with one or more heat exchangers adapted to bring the natural gas flow into heat exchange relationship with one or more refrigerant fluids, depending upon the kind of natural gas liquefaction facility 7 used.

As in FIGS. 1 and 2 , the natural gas liquefaction facility 7 is represented in FIG. 3 simply as including a refrigeration circuit comprised of a refrigeration compression system 9, a refrigerant fluid cooling arrangement including a refrigerant cooler 13 and a condenser 13A, to remove heat from the compressed refrigerant fluid, an expander 15 adapted to expand the condensed refrigerant fluid, and a heat exchanger 17. Refrigerant fluid compressed by the compression system 9 is cooled and condensed in cooler 13 and condenser 13A, expanded in expander 15 and flows in the heat exchanger 17 in heat exchange relationship with natural gas NG, to remove heat therefrom.

Thermal energy removed from the natural gas NG in the heat exchanger 17 and absorbed by the refrigerant fluid is removed from the refrigerant fluid in cooler 13 and condenser 13A and at least partly transferred, via heat pump 23 driven by electric motor 25 and relevant circuits 23A, 23B, to the thermal energy storage system 21 at a temperature higher than the temperature at which heat is discharged from cooler 13.

The natural gas NG is cooled until reaching a liquid state and liquefied natural gas LNG is collected in LNG storage and offloading facility 19, including storage tank 22 and cryogenic pump 20.

As in the previously described embodiments, also in the embodiment of FIG. 3 , thermal energy stored in storage system 21 can be used to power the gas pretreatment facility 5. A heat transfer fluid circuit 27 circulates a heat transfer fluid from the thermal energy storage system 21 to the gas pre-treatment facility 5 and from this latter back to the thermal energy storage system 21.

Similarly to the embodiment of FIG. 2 , in FIG. 3 an electric energy distribution grid 29 is electrically coupled to the electric motor 25 of the heat pump 23 and to an electric motor 31 adapted to drive a compressor 47 forming part of the compression system schematically shown at 9. It shall be noted that more than one compressor 47 and relevant electric motor drivers may be used, depending inter alia upon the liquefaction technology used.

In the embodiment of FIG. 3 , electric power is generated by a thermodynamic system 35 including a Rankine cycle. The thermodynamic system 35 comprises a circuit including one or more steam or vapor turbines, schematically represented in FIG. 3 as a single steam or vapor turbine 51. The steam or vapor turbine 51 is drivingly coupled to the electric generator 33, which is in turn electrically coupled to the electric energy distribution grid 29. The thermodynamic system 35, whereof steam or vapor turbine 51 forms part, schematically includes a boiler and evaporator 53 powered by heat from the thermal energy storage system 21 through a circuit 55, where a heat transfer fluid circulates.

In some embodiments, an additional heating unit, for example a heater and/or a superheater, can be provided to add further thermal energy to the process fluid (steam or vapor) of the thermodynamic system 35, if the thermal energy from the thermal energy storage system 21 is not sufficient. In the schematic of FIG. 3 , a superheater 57 is illustrated, which can be powered by natural gas NG from the natural gas feed 3 and/or from a boil-off gas (BOG) duct 59 from the LNG storage and offloading facility 19.

The thermodynamic circuit of steam or vapor turbine 51 further includes a condenser 59, a working fluid storage tank 60 and a pump 61.

The working fluid of the thermodynamic system 35 can be for example water, if a steam Rankine cycle is used, or an organic fluid, such as pentane, cyclopentane, carbon dioxide and the like, if an organic Rankine cycle (ORC) is used.

In order to further improve the overall energy efficiency of the system 1, in some embodiments the system 1 may further include one or more renewable energy collectors, to collect and exploit energy from renewable energy sources, such as solar energy or wind energy. In one implementation a concentrated solar power plant (CSP plant) 71 can be functionally coupled to the thermal energy storage system 21. The CSP plant 71 can include any kind of solar concentrator, for instance using heliostats, parabolic troughs, or the like. Solar energy is collected in form of heat, transferred via a heat transfer circuit 73 to the thermal energy storage system 21, and stored therein.

In further embodiments, renewable energy sources may be used to generate electric power, that in turn can be distributed, through the electric energy distribution grid 29, to one or more users or units connected thereto. In FIG. 3 a field of photovoltaic panels is shown at 77. Reference 79 indicates an inverter or a plurality of microinverters electrically coupled to the electric energy distribution grid 29 and to the photovoltaic panels 77, and adapted to convert CC electric power generated by the photovoltaic panels 77 into AC electric power. Excess electric energy may be stored in an energy storage facility 81, as such or after conversion into a different form of energy that can be stored more easily. The option is not excluded of using CC electric power generated by the photovoltaic panels 77 to produce hydrogen and to store hydrogen to be used as a fuel, for instance.

In addition to, or instead of photovoltaic panels 77, a different kind of energy collector can be used to collect energy from renewable sources, for instance wind turbines 83 of a wind farm, electrically coupled to the electric energy distribution grid 29 via inverters 85.

With continuing reference to FIGS. 1, 2 and 3 , a further embodiment of a system 1 for the liquefaction of natural gas is illustrated in FIG. 4 . The same reference numbers used in FIG. 3 designate the same or equivalent parts or elements of the system 1 of FIG. 4 , which will not be described again.

The system 1 of FIG. 4 differs from the system 1 of FIG. 3 mainly with regard to the thermodynamic system 35. In the embodiment of FIG. 4 , the thermodynamic system 35 comprises a top thermodynamic cycle 35A and a bottom thermodynamic cycle 35B, wherein the bottom thermodynamic cycle 35B uses waste heat from the top thermodynamic cycle 35A.

More specifically, the top thermodynamic cycle 35A can include an internal combustion engine. In one implementation, the top thermodynamic cycle 35A includes a gas turbine engine 91 comprised of an air compressor 93, a power turbine 95 and a combustion chamber 97. The gas turbine engine 91 can be fueled with natural gas from the natural gas feed 3 (duct 99) and/or from the BOG duct 59. The gas turbine engine 91 is drivingly coupled to an electric generator 33A, to convert mechanical power generated by the gas turbine engine 91 into electric power, which is delivered to the electric energy distribution grid 29.

Waste heat can be recovered from the flue gas of the gas turbine engine 91 in a waste heat recovery unit 100. In an implementation, the waste heat recovery unit 100 includes a waste heat recovery heat exchanger 101 (shortly referred to as WHR heat exchanger 101). A working fluid of the closed bottom thermodynamic cycle circulates in the WHR heat exchanger 101 in heat exchange relationship with the flue gas from the gas turbine engine 91. The bottom thermodynamic cycle 35B can be a Rankine cycle, preferably using an organic fluid. Pressurized working fluid is vaporized and possibly superheated in the WHR heat exchanger 101 and expanded in the vapor turbine 51, cooled and condensed in the condenser 59, collected in tank and pumped by pump 61 to the WHR heat exchanger 101 again. As in FIG. 3 , also in FIG. 4 the Rankine cycle is shown in a simplified manner, but those skilled in the art will understand that in actual fact the Rankine cycle may be a more complex cycle, including multiple superheating and/or can be a regenerative cycle.

In embodiments, further thermal power can be recovered from the flue gas in an auxiliary waste heat recovery heat exchanger 103 (auxiliary WHR heat exchanger 103) of the waste heat recovery unit 100, arranged along the flue gas path, downstream of the WHIR heat exchanger 101. Waste heat recovered in the auxiliary WHR heat exchanger 103 is delivered through a heat transfer circuit 107 to the thermal energy storage system 21.

If the temperature of the waste heat in the auxiliary WHR heat exchanger 101 is too low for direct transfer to the thermal energy storage system 21, said waste heat can be pumped to the thermal energy storage system 21 by heat pump 23 or by a further heat pump (not shown) for the same purpose.

Flue gas from the WHR heat exchanger 101 (and optionally auxiliary WHR heat exchanger 103) can be treated in a carbon dioxide capture facility 41 prior to be discharged in the environment through a stack 105, to remove carbon dioxide therefrom, and thus reduce the environmental impact of the system 1.

Carbon dioxide can be captured in the carbon dioxide capture facility 41 using any suitable post-combustion carbon capturing system or equivalent system aimed at separating and concentrating carbon dioxide generated by hydrocarbon combustion, for further uses. In embodiments of the present disclosure, thermal power from the thermal energy storage system 21 can be used to entirely or partly powering the carbon dioxide capturing facility 41. In FIG. 4 a circuit 43 is provided, wherein a heat transfer fluid circulates to transfer heat from the thermal energy storage system 21 to the carbon dioxide capturing facility 41.

The carbon dioxide capturing facility 41 can comprise carbon dioxide post treatment and export. Carbon dioxide flowing from the carbon dioxide capturing facility 41 and from the gas pre-treatment facility 5 can be gathered and transferred outside the LNG system 1 as schematically shown in FIG. 4 .

While in the previously described embodiments electric energy generated by a thermodynamic system 35 is used to drive both the heat pump 21 and the compressor arrangement 47 of the natural gas liquefaction facility 7, in other embodiments the compressor arrangement 47 can be driven by an internal combustion engine, such as a gas turbine engine, while an electric generator, driven by the same or an additional internal combustion engine, can be used to power the heat pump 23 and other ancillary equipment and facilities of the system 1.

In the above description reference has been made to a generic natural gas liquefaction facility 7, since features of the present disclosure can be beneficial in combination with any natural gas liquefaction system.

With continuing reference to FIGS. 1, 2, 3, 4 and 5 , FIGS. 6, 7, 8, 9, 10, 11, 12 and 13 illustrate schematic diagrams of natural gas liquefaction facilities using various closed or open refrigeration circuits. Each of these natural gas liquefaction facilities can be used in combination with a rejection heat recovery arrangement as described above. These circuits are generally known to those skilled in the art of natural gas liquefaction and will not be described in detail.

By way of example, FIGS. 6 and 7 illustrate exemplary embodiments of single mixed refrigerant cycles, wherein rejected thermal energy (heat) Q can be collected at the delivery side of two compressors 47A, 47B of a compression system 9, driven by a driver 31.

FIG. 8 illustrates a triple cycle, mixed refrigerant cascade system including a compression system 9 with first, second and third compressor arrangements 47A, 47B, 47C driven by drivers 31A, 31B, 31C. Each compressor arrangement includes a respective refrigerant condenser 13X, 13Y, 13Z wherefrom heat Q is rejected and wherefrom at least part of the rejected heat can be recovered via the heat pump 23 (FIGS. 1-4 ). In the schematic of FIG. 8 heat is rejected also between two sequentially arranged stages of the compressor arrangement 47C, through an intercooler 13W. At least part of the thermal energy rejected at the intercooler 13W can be recovered via the heat pump 23, as well.

FIG. 9 illustrates a cascade LNG system using three refrigeration cycles, each processing a different refrigerant and each provided with a respective compressor arrangement 47A, 47B, 47C driven by a driver 31A, 31B, 31C. Reference numbers 13X, 13Y, 13Z represent the respective refrigerant condensers, from which rejected heat Q can be recovered through the heat pump 23.

FIG. 10 illustrates a propane/mixed refrigerant LNG system including a propane compressor 47A with a driver 31A and a mixed refrigerant compressor 47B with a driver 31B. Propane condenser 13X and mixed refrigerant condenser 13Y reject heat Q which can be partly recovered through the heat pump 23.

FIG. 11 illustrates a dual-refrigerant LNG cycle similar to the cycle of FIG. 10 , with an additional refrigerant compressor 47C and respective condenser 13Z, wherefrom rejected heat Q can be collected by the heat pump 23.

FIG. 12 illustrates a dual nitrogen LNG cycle, including a plurality of nitrogen compressors 41A, 41B, 41C, 41D and respective condensers 13X, 13Y and intercooler 13W, wherefrom rejected heat Q can be recovered by the heat pump 23.

While in all the above-mentioned cycles one or more refrigerants are processed in closed circuits, LNG systems using open refrigeration circuits are known, wherein the same liquefied natural gas is used as the refrigerant. Rejected heat recovery via the heat pump 23 can be provided also in such open-circuit LNG systems. An exemplary open circuit using LNG as refrigerant for natural gas liquefaction is shown schematically in FIG. 13 . Two compressors 47A, 47B, a condenser 13Y and an intercooler 13W are schematically illustrated in FIG. 13 . Rejected heat Q can be recovered from the condenser 13Y and from the intercooler 13W.

In addition to recovering low-temperature thermal energy rejected from the refrigerant coolers of the refrigeration circuit, in some embodiments, low-temperature thermal energy can be recovered also from other low-temperature heat sources of the system 1, for instance from the pre-treatment section 5 during batchwise regeneration cycles performed therein.

While the invention has been described in terms of various specific embodiments, it will be apparent to those of ordinary skill in the art that many modifications, changes, and omissions are possible without departing form the spirit and scope of the claims. In addition, unless specified otherwise herein, the order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments. 

1. A natural gas liquefaction system, comprising: a natural gas feed; a refrigeration circuit including: a refrigeration compressor adapted to compress at least one refrigerant fluid; a refrigerant cooler adapted to remove low-temperature thermal energy from the refrigerant fluid; a driver adapted to drive the refrigeration compressor; a heat exchanger adapted to receive a flow of natural gas from the natural gas feed and remove heat therefrom through heat exchange against refrigerant fluid; and, a heat pump adapted to recover low-temperature thermal energy rejected from the natural gas liquefaction system and to transfer the rejected thermal energy to a thermal energy storage system at a temperature higher than the temperature at which the thermal energy has been rejected.
 2. The system of claim 1, wherein the heat pump is adapted to recover thermal energy rejected by the refrigerant cooler.
 3. The system of claim 1, further including at least one processing facility, powered by thermal energy from the thermal energy storage system.
 4. The system of claim 3, wherein the at least one processing facility; comprises a gas pre-treatment facility, adapted to receive raw natural gas and pretreat said raw natural gas prior to deliver the natural gas to the natural gas liquefaction facility.
 5. The system of claim 1, wherein the driver comprises at least one electric motor; and further comprising at least one electric generator, configured to generate electric energy to power the electric motor, and a thermal energy conversion system adapted to convert thermal energy into mechanical energy and to drive the at least one electric generator therewith.
 6. The system of claim 5, wherein the thermal energy conversion system comprises an internal combustion engine, preferably a gas turbine engine, fueled with natural gas directly or indirectly delivered by the natural gas feed.
 7. The system of claim 6, further comprising a waste heat recovery unit adapted to recover waste heat from the internal combustion engine and transfer waste heat to the thermal energy storage system.
 8. The system of claim 6, wherein the at least one processing facility comprises a carbon dioxide capturing facility adapted to receive flue gas from the internal combustion engine and remove carbon dioxide therefrom.
 9. The system of claim 5, wherein the thermal energy conversion system comprises a thermodynamic circuit adapted to receive thermal energy from the thermal energy storage system.
 10. The system of claim 9, wherein the thermodynamic circuit comprises a steam or vapor turbine drivingly coupled to the at least one electric generator.
 11. The system of claim 6, wherein the thermal energy conversion system further comprises a low-temperature thermodynamic circuit configured to receive waste heat from the internal combustion engine; wherein preferably the low-temperature thermodynamic circuit comprises a steam or vapor turbine drivingly coupled to the at least one electric generator.
 12. The system of claim 5, wherein the at least one electric generator is electrically connected to at least one of: the heat pump; and the at least one processing facility.
 13. The system of claim 5, wherein the at least one electric generator is functionally coupled to an energy storage facility, adapted to store a surplus energy generated by the at least one electric generator.
 14. The system of claim 1, further comprising a renewable energy collector adapted to collect energy from a renewable energy source.
 15. The system of claim 14, wherein: the renewable energy collector is adapted to convert energy from the renewable energy resource into a storageable energy, preferably into one of: thermal energy and electric energy; and the renewable energy collector is functionally coupled to at least one of said thermal energy storage system and an additional energy storage system.
 16. The system of claim 1, further comprising a liquefied natural gas storage and offloading facility; and wherein the liquefied natural gas storage and offloading facility is powered by electric energy generated by the at least one electric generator.
 17. A modular skid, comprising: a refrigeration circuit configured to couple with a natural gas feed, the refrigeration circuit including: a refrigeration compressor adapted to compress at least one refrigerant fluid; a refrigerant cooler adapted to remove low-temperature thermal energy from the refrigerant fluid; a heat exchanger adapted to receive a flow of natural gas from the natural gas feed and remove heat therefrom by heat exchange with the refrigerant fluid; a heat pump adapted to be coupled to a source of low-temperature rejected thermal energy and to couple with a thermal energy storage system, the heat pump adapted to collect rejected low-temperature thermal energy and transfer the rejected thermal energy to the thermal energy storage system at a higher temperature.
 18. The modular skid of claim 17, wherein a cold side of the heat pump is configured to recover heat from the refrigerant cooler.
 19. A method for liquefying natural gas with a natural gas liquefaction system, the natural gas liquefaction system including: a heat exchanger; a refrigeration compressor adapted to compress a refrigerant fluid; a refrigerant cooler adapted to remove low-temperature thermal energy from the refrigerant fluid; a driver adapted to drive the refrigeration compressor; the method comprising the following steps: flowing natural gas in the heat exchanger and removing thermal energy from the natural gas by heat exchange against the refrigerant fluid; removing low-temperature thermal energy from the refrigerant fluid through the refrigerant cooler; and recovering low-temperature thermal energy rejected from the natural gas liquefaction system and transferring the rejected thermal energy to a thermal energy storage system at a higher temperature through a heat pump.
 20. The method of claim 19, wherein the step of recovering low-temperature thermal energy rejected from the natural gas liquefaction system comprises the step of recovering low-temperature thermal energy removed from the refrigerant fluid through the refrigerant cooler.
 21. The method of claim 19, further comprising the step of delivering thermal energy from the thermal energy storage system to at least one processing facility; of the natural gas liquefaction system.
 22. The method of claim 21, wherein the at least one processing facility includes a gas pre-treatment facility, and wherein the method further comprises the step of treating a flow of raw natural gas in the gas pre-treatment facility prior to introducing the natural gas in the heat exchanger.
 23. The method of claim 22, wherein the step of recovering low-temperature thermal energy rejected from the natural gas liquefaction system comprises the step of recovering thermal energy rejected from the gas pre-treatment facility.
 24. The method of claim 19, wherein the driver comprises an electric motor, the method further comprising the following steps: converting thermal energy into mechanical energy in at least one thermodynamic cycle; converting mechanical energy into electric energy with at least one electric generator, and powering said electric motor therewith.
 25. The method of claim 24, wherein the step of converting thermal energy into mechanical energy comprises the step of powering an internal combustion engine, preferably a gas turbine engine, with natural gas; and wherein the step of converting mechanical energy into electric energy comprises the step of driving the at least one electric generator with said internal combustion engine.
 26. The method of claim 25, further comprising the step of capturing carbon dioxide from flue gas generated by the internal combustion engine in a carbon dioxide capturing facility powered with thermal energy from the thermal energy storage system.
 27. The method of claim 25, further comprising the step of collecting waste heat from the internal combustion engine and further converting said waste heat in mechanical energy with a low-temperature thermodynamic cycle.
 28. The method of claim 25, further comprising the step of collecting waste heat from the internal combustion engine and transferring said collected waste heat to the thermal energy storage system.
 29. The method of claim 24, further comprising the step of powering the heat pump with electric energy generated by the at least one electric generator.
 30. The method of claim 19, further comprising the step of collecting energy from a renewable energy source; and further comprising at least one of the following steps: storing said collected energy; or converting said collected energy into electric energy and using the electric energy in the at least one processing facility.
 31. The method of claim 30, wherein the step of storing said collected energy comprises the step of converting said collected energy into thermal energy and storing the converted thermal energy in the thermal energy storage system. 